1. Field of the Invention
The present invention relates to a film bulk acoustic resonator (FBAR) device and a method for fabricating the film bulk acoustic resonator (FBAR) device, and more particularly to a film bulk acoustic resonator (FBAR) device which comprises a membrane layer having a plurality of active regions and provides different resonant frequencies by providing different thicknesses of the active regions of series resonators and shunt resonators and by controlling the thicknesses, respectively, and a method for fabricating the film bulk acoustic resonator (FBAR) device.
2. Description of the Related Art
Recently, components for a mobile communication terminal such as radio frequency (RF) components have been developed to satisfy the trends of miniaturization and high-functionality of mobile communication terminals. Among the radio frequency (RF) components for mobile communication terminals, a filter is an essential component and serves to select a desired signal from many sky waves or to filter specific signals.
Particularly, as usable frequency bands of the mobile communication terminal increase, super high frequency (SHF) devices are required more and more. However, with the super high frequency (SHF) devices, it is difficult to satisfy the miniaturization and the low-cost trend required in mobile communication terminals. For example, a resonator or a filter which is operated at a frequency of more than 1 GHz has a large size, thereby not being integrated. Further, a dielectric resonator is replaced with a crystal resonator or a surface acoustic resonator. However, the crystal resonator and the surface acoustic resonator increase an insertion loss and cannot accomplish integration, miniaturization, and low cost.
In order to solve the aforementioned problems, a film bulk acoustic wave resonator (“FBAR”, or may be referred to as a thin film resonator “TFR”) using the resonance of a piezoelectric layer, has been developed and is now practically used.
Conventionally, the film bulk acoustic wave resonator (FBAR) device is a thin film type device, in which a thin film made of piezoelectric material such as ZnO or AlN is formed on a semiconductor substrate made of silicon or GaAs, thereby inducing resonance by piezoelectricity of the thin film. The film bulk acoustic wave resonator (FBAR) device is low-priced, miniaturized, and has a high quality. The film bulk acoustic wave resonator (FBAR) device can be used in radio communication equipment at various frequency bands (900 MHz˜10 GHz) and military radar. Further, the film bulk acoustic wave resonator (FBAR) device can be micro-miniaturized to have a size less than one hundredth of that of a dielectric filter or a lumped constant (LC) filter, and has an insertion loss less than that of a surface acoustic wave device.
FIG. 1a is a circuit diagram of a conventional film bulk acoustic resonator (FBAR) device for a thin film filter, and FIG. 1b is a graph of the conventional film bulk acoustic resonator (FBAR) device for the thin film filter. FIG. 1a shows the ladder type conventional film bulk acoustic resonator (FBAR) device comprising a series resonator 11 and a shunt resonator 12 connected to the series resonator 11. That is, a film bulk acoustic resonator (FBAR) filter is fabricated by the composition of the series resonator 11 and the shunt resonator 12. Herein, different resonant frequencies of the series resonator 11 and the shunt resonator 12 are determined by the total thickness of a stack structure of the resonator including a piezoelectric layer. The difference of center frequencies between the series resonator 11 and the shunt resonator 12 is obtained by the difference of thicknesses of upper surfaces of the series resonator 11 and the shunt resonator 12. Herein, the stack structure of the resonator determines the resonant frequency of the resonator, and usually comprises a lower electrode, the piezoelectric layer, and an upper electrode.
That is, the film bulk acoustic resonator (FBAR) device resonates due to the coupling between mechanical stress and load on the piezoelectric layer. The resonant frequency has a length of half an acoustic wave progressing within the device having the same thickness of the stack structure of the resonator comprising the piezoelectric layer and the electrodes. Therefore, the resonant frequency is determined by the total thickness of several layers of the stack structure of the resonator comprising the lower electrode, the piezoelectric layer, and the upper electrode.
The aforementioned film bulk acoustic resonator (FBAR) filter is shaped in a ladder type. The ladder type film bulk acoustic resonator (FBAR) filter is disclosed by “Thin Film Bulk Acoustic Wave Filters for GPS by K. M. Lakin et al. (Lakin), IEEE Ultrasonic Symposium, 1992, pp.471-476”. According to this paper, the ladder type film bulk acoustic resonator (FBAR) filter is formed by the composition of the series resonator and the shunt resonator. Each of the series resonator and the shunt resonator has a different center frequency.
Further, the film bulk acoustic resonator (FBAR) device comprises the lower electrode, the piezoelectric layer, and the upper electrode, which are formed on the silicon substrate in order. Herein, an electric field is applied to the upper and the lower electrode, and then a bulk acoustic wave is generated on the piezoelectric layer. In order to maintain the high quality (high-Q), the bulk acoustic wave must be prevented from affecting the silicon substrate. Therefore, in order to prevent the bulk acoustic wave from affecting the silicon substrate, a structure for separating the piezoelectric layer from the silicon substrate is required. Electrical characteristics the film bulk acoustic resonator (FBAR) device such as an insertion loss and a transducer gain, and a practical use of the film bulk acoustic resonator (FBAR) device depend on the aforementioned separation structure. The separation structure is mostly divided into a reflecting layer structure using Bragg reflection and an air-gap structure. The reflecting layer structure is very complicated and takes long time to manufacture. Further, compared to the air-gap structure, the reflecting layer structure deteriorates an insertion loss and reflective characteristics and reduces effective bandwidth. Therefore, the air-gap structure has been used now.
FIG. 2 is a cross-sectional view of the conventional film bulk acoustic resonator (FBAR) device. With reference to FIG. 2, the conventional film bulk acoustic resonator (FBAR) device 20 comprises a piezoelectric layer 22, which is supported by an outer circumference of a well 24 formed on an upper surface of a substrate 26. An upper electrode 28 and a lower electrode 30 are formed on the upper and lower surfaces of the piezoelectric layer 22. The upper electrode 38 and the lower electrode 30 correspond to the well 24. The electrical connection between the upper electrode 38 and the lower electrode 30 is accomplished by an upper terminal 36 and a lower terminal 38. A stack structure 32 of the conventional film bulk acoustic resonator (FBAR) device 20 is formed by the piezoelectric layer 20, the upper electrode 28 and the lower electrode 30. Herein, the stack structure 32 of the film bulk acoustic resonator (FBAR) device 20 expands or contracts in the direction of the arrow 34 according to a direction and a level of a voltage provided between two electrodes 28 and 30.
As described above, in the conventional film bulk acoustic resonator (FBAR) device 20, a center frequency of the resonator depends on the total thickness of the stack structure 32 comprising the piezoelectric layer 20, the upper electrode 28 and the lower electrode 30. Therefore, in order to provide different center frequencies, the thickness of the upper electrode 28 or the lower electrode 30, or the thickness of the piezoelectric layer 20, must be adjusted.
Further, it is difficult to finely control the resonant frequency by the adjustment of the thicknesses of the electrodes or the piezoelectric layer. Moreover, it is not easy to simultaneously manufacture a transmission filter and a reception filter on the same substrate.